1. Field of the Invention
The present invention relates to an inverse telescopic wide angle lens, and more particularly to an inverse telescopic wide angle lens capable of providing an ultra wide image angle adapted for use as a photographic lens and having a rear focal length larger than the focal length of the lens system.
2. Related Background Art
In the field of inverse telescopic wide angle lens having a front lens group of a negative power in front of a rear lens group of a positive power, there have been made various proposals and various types are already known, as exemplified by a configuration disclosed in the U.S. Pat. No. 3,663,095. In such inverse telescopic wide angle lens, the negative refractive power of the front lens group is required for obtaining a rear focal length larger than the focal length, but has the effect of increasing the image angle to the rear positive lens group, thus bringing the curvature of image plane and the Petzval's sum to the negative side. However such negative refractive power can improve the lens performance in a lens system in which the ratio of the rear focal distance to the focal length is not so large.
Nevertheless, in an ultra wide angle lens with an image angle of 80.degree., 100.degree. or even larger in which the distortion aberration, astigmatism or coma aberration is difficult to correct because of the asymmetric structure of the lens system itself and a relatively long rear focal length is required for this reason, the undesirable effect of the negative refractive power of the front lens group appears evidently in the excessively negative distortion and Petzval's sum and in the distortion in the coma aberration. Also in general, in order to minimize the negative distortion generated by a negative meniscus lens generally contained in the front lens group of negative retractive power, also to maintain a satisfactory image plane and to reduce the spherical aberration of the pupil, the lens bendings have to be so designed that the principal ray passes through the negative meniscus lens at about the minimum deviation angle.
However, a negative coma aberration is also generated by the concave face of the negative lens. In particular a large negative coma aberration is generated for the lights outside the principal ray, and the correction therefor becomes a significant burden for the lens designing. On the other hand, a modification in the lens bending for reducing said negative coma aberration will result in other drawbacks such as an increased distortion.
On the other hand, the aberrations other than the distortion can be reduced by increasing the dimension of the lens system, particularly the front lens group. Such proportional enlargement of the lens allows to use the light relatively close to the principal ray, thereby reducing the coma aberration. However such method is basically incapable of correcting the distortion aberration, because the distortion is represented by the ratio of the height of the ideal image point to that of the actual image point and does not change by the proportional enlargement of the lens is the image angle remains same, unless the path of the principal ray is varied. Thus the proportional enlargement of the lens is basically ineffective for the correction of distortion, though it is effective for the correction of coma or image plane curvature. This fact will be readily understood by a wide angle conversion system, which, when magnified in dimension, can improve the spherical aberration and the coma aberration though it does not vary the magnification or the distortion.
However such proportional enlargement of the lens tends to render the lens bulky, though the ease of correction of coma and image plane curvature is advantageous for the correction of distortion in the lens designing. Also a positive lens is often placed close to the negative lens for the purpose of correcting the distortion, and thus configuration tends to further increase the dimension of the lens system.
Also the use of an aspherical face (i.e., surface) for correcting the distortion is already known, and there is already known a system employing an aspherical convex face in the negative meniscus lens of the front lens group of negative refractive power, for the purpose of said correction. Although such configuration is effective to a certain extent, such aspherical face only plays an auxiliary role in the optical system consisting of spherical faces only, and is insufficient when the image angle becomes wider. For example a lens system with an image angle of 100.degree. or larger is difficult to design, and the configuration of the lens system becomes inevitably complex by the above-mentioned conventional methods. Also the dimension of the lens tends to become larger in the axial and radial directions.
On the other hand, the focusing of a phototaking lens has generally been conducted by the movement of the entire lens system, but there are also known other methods, such as a method of moving a front lens group, positioned closest to the object, as employed in zoom lenses, and a method of fixing the front lens group relative to the image plane and moving a rear lens group together with a diaphragm, as employed in telephoto lenses.
However, the method of moving the entire lens system or the front lens group is associated with draqbacks of requiring a large and complex moving mechanism and of a slow response speed in focusing. Also the method of moving the rear lens group is not free from the drawbacks of complex lens moving mechanism and slow response in focusing, because an automatic diaphragm control mechanism is recently attached to the diaphragm, for achieving automatic exposure.
In this connection, in the conventional inverse telescopic wide angle lens, there is known a focusing method of moving the entire lens system and simultaneously varying appropriate one of the air gaps separating the component lenses. However such focusing method is equivalent to the conventional general focusing method in that the entire lens system has to be moved. Also the variation in the air gap separating the component lenses is to correct the movement of the image plane to the positive direction when the lens is focused to an object of a short distance, and is not intended for focusing.